Issue

Semiconductor industry tackles nanotube contamination issues

01/01/2005

By Hank Hogan

AUSTIN, Texas-Uniting those who use nanotechnology with those who create it, NanoForum 2004, held here in mid-November, may become an annual meeting focused on moving nanotechnology from an academic endeavor to a commercial reality. And that transition, participants agree, is going to have a significant impact on cleanrooms as well as the future of contamination control processes and procedures in a variety of manufacturing arenas.

“No conference has focused on the tools and materials required to really bring this technology to full commercial fruition,” said conference chair Ellery Buchanan, explaining why NanoForum concentrated on manufacturing and markets. More than 400 conferees from the U.S., Europe, and Asia took part in the SEMI-sponsored (www.semi.org) event.

A major concern for nanotechnology contamination control involves new materials with unique properties, such as carbon nanotubes. Under the proper conditions, a sheet of carbon atoms rolls up to form nanometer diameter tubes, or nanotubes. While only nanometers in width, nanotubes can be hundreds of microns long.

Thanks to their regular arrangement of atoms held together by sturdy carbon-carbon bonds, nanotubes are resistant to chemicals and heat. They’re also strong, conduct electricity well, are small, and have a high aspect ratio. This unique combination of characteristics makes nanotubes potentially the ultimate reinforcement fibers for composites as well as offering the ability to build nanotools and conducting and other nanowires. Nanotubes are showing up in semiconductor memories, bone scaffolding, and video displays.

A wide range

A typical nanotube manufacturing process involves the catalytic decomposition of a carbon containing reaction gas such as acetylene or methane, with iron, cobalt or nickel used as a catalyst. Because nanotubes are relatively new, the manufacturing kinks-and potential markets-are still being worked out.

Raymond McLaughlin is executive vice president and CFO at Carbon Nanotechnologies, Inc. (Houston, Texas; www.cnanotech.com), a producer that supplies nanotubes to a wide range of customers. As such, the company deals with customers with varying purity needs in the bulk nanotubes they buy. McLaughlin notes that his company’s nanotube manufacturing isn’t done in a cleanroom but rather in a closed chamber. The process is controlled to create nanotubes of the desired characteristics and to ensure that unwanted contaminants aren’t created or creep in later. McLaughlin, however, doesn’t believe unusual contamination control measures are required.

“We take normal contamination precautions, as with any chemical or polymer,” he says. “We want to make sure our customer gets the product he bargained for and that requires close control of handling and packaging.”

Memory revolution

While some nanotubes can end up in goods as common as tennis rackets, others make their way into more critical cleanroom-produced products. For example, Nantero, Inc. (Woburn, Mass.; www.nantero.com) has pioneered a technology that uses nanotubes in semiconductor memories where the flexible rods act as storage elements. Suspended between two supports, the nanotubes stretch across a microscopic gap in one of two configurations: a straight shot or a downward bow. One arrangement, the straight shot, for example, is considered a zero while the other is a one. In this way, data can be stored at each location based on the configuration of the nanotubes. Nantero touts its nonvolatile random access memory (NRAM) as a universal memory solution because NRAM devices combine the speed, density, and nonvolatility that are only found today in the three separate memory technologies: DRAM, SRAM, and flash.

NRAM technology is being implemented by Nantero and partner LSI Logic Corp. (Milpitas, Calif.; www.lsilogic.com). The two have incorporated nanotubes into LSI Logic’s semiconductor processes with a goal of producing megabit NRAM on chips by the second half of 2006. Norm Armour, vice president and general manager of Gresham operations for LSI Logic, says that from generation to generation, his company’s system- on-a-chip devices contain an increasing amount of memory. Having NRAMs on board may mean that other stand-alone memory devices and chips could be eliminated. “That would be a huge boon for us,” says Armour.

But for that to happen, the nanotubes must become a part of a semiconductor device with all that implies for cleanliness, reliability, and contamination control. According to Thomas Rueckes, Nantero’s chief scientific officer and co-founder, bulk nanotubes from any commercial vendor contain trace elements of the metallic catalysts and many carbon contaminant byproducts of manufacturing. In a semiconductor process, the result of using standard nanotubes is molecular contaminants and particles-a lot of particles. “If you just buy nanotubes from any of the vendors, you will have particle levels of hundreds of millions,” Rueckes claims.

Nantero had to develop a way to reduce the metallic contaminants to a parts-per-billion (ppb) level and find a way to remove the particles. What’s more, the company had to do so while developing a means to disperse the nanotubes in a semiconductor process-compatible solvent.

All three of these goals have been met-achievements that Nantero and LSI Logic say are worldwide firsts. The key is that nanotubes are treated like other semiconductor materials. Thus, Rueckes says nanotubes are dispersed in a solvent over a wafer. Subsequent photolithographic processing selectively removes the nanotubes, leaving them only in the desired locations. To reach the contamination control targets with regard to trace metals and particulates, Nantero uses a proprietary sequence of tightly controlled processing steps, such as filtration, in a series of different solvents.

Life science benefits

In the case of bone scaffolding, commercial prospects for nanotube use are not immediate, but research indicates that nanotubes can mimic bone and potentially allow cells to create replacement bone and joints. Thomas Webster, an assistant professor of biomedical engineering at Purdue University (W. Lafayette, Ind.; www.purdue.edu) announced in late November promising bone scaffolding results. Webster notes that the nanotubes used in the research were purified to remove trace contaminants. Metallics, for instance, were vacuumed out using a magnet while a soak in an acid, HCL, chemically removed contaminants.

Nantero’s NRAM nanotube technology for semiconductor memories serves as a universal memory solution, combining DRAM, SRAM and flash technologies. Nanotubes stretch across a microscopic gap in either straight shot or downward-bow configurations, and data can be stored at each location based on the configuration of nanotubes. But to achieve those results, the challenge has been to reduce metallic contaminants to parts-per-billion and to remove molecular particles. Source: Nantero, Inc.

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“We did it to ensure that they could be aligned properly and that the cells were just interacting with the carbon on the tubes,” says Webster. He adds that the process also included sterilization to ensure no harmful bacteria were present. While results are promising, Webster thinks it will be years before this research emerges from a lab.

When nanotubes do become incorporated into standard manufacturing processes, whatever contaminants they possess will have to be considered. In some cases, such as the bone scaffolding and other tissue engineering, it may be possible for bulk nanotube manufacturers to supply the product with the required cleanliness by doing some type of post-production processing. In others, such as semiconductor applications, that may never be possible. For that situation, those incorporating nanotubes will have to devise some way to clean them up, as did Nantero and LSI Logic.

In either case, researching, supplying or integrating these new materials may take place in a semiconductor or pharmaceutical grade cleanroom. That means there’ll be more demand for contamination control and possibly new processes required to achieve desired purity levels. It’ll all be a part of the increasing use of nanotechnology in general. SEMI’s Buchanan says, “From a cleanrooms perspective, there’s going to be more and more of these facilities.